In a wireless communication system, an example of a technique for limiting a bandwidth of a transmission/reception signal without causing a signal distortion includes a method using a root Nyquist filter. Hereinafter, a wireless communication system comprising a root Nyquist filter mentioned in Non-Patent Document 1 (“Modulation and Demodulation of Digital Wireless Communication” by Yoichi Saito, Institute of Electronics, Information and Communication Engineers, p. 47-57) will be described using FIG. 1.
FIG. 1 is a block diagram showing a configuration of a related wireless communication system.
As shown in FIG. 1, the wireless communication system comprises transmitter 801 and receiver 802.
Transmitter 801 comprises transmission symbol generator 103, pilot generator 104, signal multiplexer 105, root Nyquist filter 803, and variable gain amplifier 107.
Transmission symbol generator 103 generates transmission information to perform symbol mapping and outputs transmission symbol sequence STXS. Pilot generator 104 generates pilot symbol SPI. Signal multiplexer 105 multiplexes transmission symbol sequence STXS and pilot symbol SPI and outputs multiplexed signal SMUX. Root Nyquist filter 803 applies filtering with root roll-off characteristics to multiplexed signal SMUX and outputs filtering transmission signal SFTX. Variable gain amplifier 107 amplifies filtering transmission signal SFTX in accordance with channel quality information SCQI and outputs the signal as transmission signal STX.
Receiver 802 comprises root Nyquist filter 804, signal separator 109, data reproducer 110, and channel quality estimator 111.
Root Nyquist filter 804 applies filtering with root roll-off characteristics to reception signal SRX corresponding to transmission signal STX and outputs filtering signal SFRX. Signal separator 109 separates filtering signal SFRX into reception symbol sequence SRXS and reception pilot signal SRXPI. Data reproducer 110 uses reception symbol sequence SRXS and reception pilot signal SRXPI to reproduce transmission data and outputs the data as reproduction data SRED. Channel quality estimator 111 estimates a channel quality from reception pilot signal SRXPI and outputs the estimation result as channel quality information SCQI.
In the wireless communication system shown in FIG. 1, transmitter 801 and receiver 802 use root Nyquist filters 803 and 804 to apply filtering to the transmission/reception signal, thereby limiting a signal band by roll-off characteristics as shown in FIG. 2, without causing signal distortion.
In the related wireless communication system, as shown in FIG. 2, the smaller the roll-off rate α (0<α<1), the more sharply can the signal band be limited so that use efficiency of the radio frequency is improved. However, if the roll-off rate is made small, a PAPR (Peak to Average Power Ratio), which a ratio of peak power to average transmission power, becomes large. Therefore, transmission power needs to be limited to avoid signal distortion due to saturation of a transmission amplifier (variable gain amplifier shown in FIG. 1). Thus, there is a problem in which the reaching distance of the transmission signal becomes short and the communication range becomes narrow.